Rigging objects to Second Life Armature Issue
I have somehow fixed my issue. 

I re-opened Blender and started completely over. 
Staying in scene one, I uploaded the Second Life Avatar. Then I opened the foot file I wanted to attach and rig. I realigned the foot to match exactly vertex for vertex again. 

Once perfectly aligned, I pressed shift + D to duplicate it in case I somehow mangled it. I moved this to a new scene. 

In the new scene, I placed a box inside my foot so that I could apply a mirror modifier to create the opposite foot. I then went into edit mode and chose faces and deleted the cube faces inside the foot so that it won't be seen in Second Life. I took a copy of the mirrored foot into scene one and aligned it to the SL Avatar.

I then chose both feet, rigged them to the armature by pressing Control + P and chose Deform with empty vertex groups.

Lastly, I chose the legs of the avatar (which included the original feet) and then chose the feet, changed to weight paint mode and chose transfer weights.

I checked the rig by going into pose mode and rotated the feet to make sure it was all working correctly. It was!